Subthreshold slope and transconductance

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In summary, subthreshold slope refers to the change in drain current relative to the change in gate voltage, while transconductance is the proportionality between drain current and gate voltage minus the threshold voltage. The subthreshold slope is proportional to the transconductance, and both have physical significance in the behavior of a MOSFET. Transconductance is related to the device's exponential increase in drain current above the cutoff gate voltage, while the subthreshold slope is related to the rapid decrease in current as bias is reduced below the cutoff point.
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ZeroFunGame
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I'm trying to understand the difference between subthreshold slope and transconductance. The subthreshold slope is the change in drain current / change in gate voltage. The transconductance of a MOSFET is proportional to the drain current / VGS - Vth. It would seem the subthreshold slope is proportional to the transconductance? I'm trying to understand the physical significance and distinctions between subthreshold slope and transconductance. Thank you for any thoughts
I'm trying to understand the difference between subthreshold slope and transconductance. The subthreshold slope is the change in drain current / change in gate voltage. The transconductance of a MOSFET is proportional to the drain current / VGS - Vth. It would seem the subthreshold slope is proportional to the transconductance? I'm trying to understand the physical significance and distinctions between subthreshold slope and transconductance. Thank you for any thoughts
 
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As far as I can ascertain (regarding myself as a vacuum tube engineer!), the MOSFET has a gate voltage corresponding to cut-off, where the drain current is nominally zero. Above this gate voltage the drain current increase exponentially - it is like a square law device. The transconductance is the slope when biased above cut off and in normal operarting conditions.
For gate voltages below cut off, the drain current is not actually zero but is small. As bias is reduced further, the current reduces rapidly and if plotted as a log it is a straight line. This is similar to a diode. The slope of this line is the sub threshold slope.
 
  • #3
Ah, so transconductance is always the relation above Vth for a MOSFET!

tech99 said:
increase exponentially - it is like a square law device

increases quadratically?
 

1. What is subthreshold slope?

Subthreshold slope is a measure of the steepness of the current-voltage curve of a transistor in the subthreshold region. It is typically expressed in units of volts per decade and is an important factor in determining the energy efficiency of a transistor.

2. How is subthreshold slope related to transconductance?

Transconductance is the ratio of the change in output current to the change in input voltage of a transistor. In the subthreshold region, the transconductance is inversely proportional to the subthreshold slope. This means that a lower subthreshold slope results in a higher transconductance and vice versa.

3. What factors affect subthreshold slope?

Subthreshold slope is affected by various factors such as the doping concentration of the transistor, the gate oxide thickness, and temperature. Higher doping concentration and thinner gate oxide result in a lower subthreshold slope, while higher temperature can increase the subthreshold slope.

4. Why is subthreshold slope important in low-power applications?

In low-power applications, reducing the subthreshold slope is crucial for improving energy efficiency. A lower subthreshold slope means that the transistor can switch between on and off states more efficiently, resulting in less power consumption.

5. How is subthreshold slope measured?

Subthreshold slope can be measured by plotting the current-voltage curve of a transistor in the subthreshold region and calculating the slope of the curve. It can also be calculated using the formula: S = (kT/q) * (dV/dlog(I)), where k is Boltzmann's constant, T is temperature, q is the electron charge, V is the voltage, and I is the current.

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